The measurement of automobile exhaust flow poses many problems. At high flows the gas temperature is high, reaching up to 1200° F. for high speed, high load conditions. At idle, the temperature is lower, but the flow pulsates, and in some cases actually reverses between the gas out flow bursts. This condition is more pronounced for four cylinder engines. In addition, the dew point of the exhaust gas is in the order of 150 to 170° F. In emission measurements, where it is necessary that the chemical composition of the exhaust gas not be changed, it is necessary for the measurement devices to be heated to above the gas dew point.
The pulsating nature of the exhaust at idle conditions poses the most severe problems to current measurement systems. To understand the nature of this flow, consider the automobile engine. After the cylinder fires, the exhaust valve opens and the piston expels the gas. Then the valve closes; however, the inertia of the flowing gas continues to push the gas out the tail pipe, while creating a vacuum in the exhaust manifold. As the force of the vacuum overcomes that of the gas inertia, the flow reverses, pulling some gas back into the exhaust manifold and, under certain conditions, may pull outside air back into the tail pipe. As the engine speed increases, the exhaust valve openings occur closer together and eventually overcome the time constant of the exhaust system. Engine manufacturers make use of this effect to increase low end torque by “tuning the exhaust ports”. Obviously, this effect is more pronounced for 4 cylinder engines than for 6 or 8 cylinder engines.
The majority of the measuring systems use unidirectional flow measurement devices. Since many of these devices sense pressure changes across a orifice or nozzle, the outputs of which are proportional to the square of the gas flow rate, pulsating unidirectional flow alone can cause errors, since the average pressure is not proportional to the square of the average gas flow. Other devices which do not have the non-linearity of the differential pressure can be used, but if there is flow reversal during the exhaust cycle, these devices measure the outgoing flow, then measure it a second time when the reversed gas exits for the second time. Typical errors due to this problem can run as high as 40 to 50%. Examples of this type of meter and its performance under idle conditions are described in the following paragraphs.
One gas flow sensor is the Vortex meter which consists of a non-streamlined strut held in the flow stream. As the flow passes this strut, vortices are formed and are shed behind the strut. Downstream a short distance is an ultrasonic beam, which intercepts the vortices as they pass. Descriptions of this measuring technique are contained in Joy et al U.S. Pat. No. 3,680,375, Joy U.S. Pat. No. 4,437,349, Joy et al U.S. Pat. No. 4,240,299, Joy et al U.S. Pat. No. 3,979,309, and in the Society of Automotive Engineers publication “Vortex flowmeter applications to automobile engine control”, R. D. Joy, 1975.
This type of sensor produces a sine wave type output, with the frequency of the sine wave being linearly proportional to the volumetric flow rate of the gas and independent of the gas composition. An example of the sensor output under steady flow is shown in FIG. 1 of the drawings.
However, under pulsating exhaust flow of a 2.0 liter four cylinder engine, the sensor output is shown in FIG. 2. The engine was running at about 750 RPM, and with two cylinder firings per revolution, there would be about 25 exhaust bursts per second. The scale on the second plot runs from 0 to 80 milliseconds, so the two bursts represent two exhaust valve openings. In between the two bursts are some indication of reverse flow occurring. Counting the zero crossings shown in this figure indicates a flow rate of about 30 cubic feet per minute (CFM). However, it was known that the actual exhaust rate should be between 5 and 10 CFM.
In the past, a common practice to measure the low flow was to run the exhaust flow through one or two barrels, which, combined with the flow resistance in the connecting pipes, created a smoothing filter. With this technique, the measurement device is placed at the exit of the last barrel, where the flow is smooth and continuous. While this approach can be used to measure the low speed exhaust rate, it has the disadvantages of imposing a time lag of several seconds on the flow, which prevents measurement of rapid flow changes. Also, because the barrels and connecting pipes were difficult to heat, condensation occurred, changing the chemical analysis of the gas, which analysis is required under EPA vehicle certification. Consequently, the automobile industry was forced to collect the exhaust gas in large plastic bags, and, to prevent condensation, to add dry air of a known amount to lower the dew point of the mixture below ambient temperatures.
While this has worked for several years, it has become difficult to handle because the added dry air in many cases is now more “dirty” than the exhaust gas emitted by the engine. The industry is now developing a “clean air system” to remove the hydrocarbons and nitrous oxide compounds.
The test method preferred by both the automobile industry and the EPA is to directly measure the exhaust flow rate without allowing condensation, and withdrawing a small gas sample proportional to the flow rate for analysis. Then, using a dynamometer, the engine is put through a simulated driving cycle containing a number of rapid accelerations, which must be followed by the exhaust flow device. There is therefore a need for an improved technique for measuring automobile exhaust flow.